Research reports reveal Kaikoura tsunami hazard

Significant
coastal inundation as well as flooding in parts of Kaikoura
township are likely if a large South American earthquake
sets off a tsunami. This and other findings are included in
the latest report on Tsunami and Inundation Modelling for
Kaikoura and North Canterbury released today by Environment
Canterbury,

At the same time, Environment Canterbury is
releasing a report on Kaikoura’s Landslide Tsunami Hazard.
This report covers Phase-1 of a study into the risk of
tsunamis generated by landslides from the unstable sea floor
at the top of the Kaikoura Canyon. It does not include any
modelling of new landslide-tsunami events, which will happen
in the next two phases of the research due for completion
later this year and next year.

Both reports are part of a
region-wide tsunami-hazard investigation involving a
partnership between Environment Canterbury, NIWA, and
scientists from the Universities of Canterbury and Bremen.
The Kaikoura District Council and Canterbury Civil Defence
Emergency Management Group have also been
involved.

“There is a risk to the Kaikoura township and
along the coast, especially around the Clarence River Mouth
as far as Kekerengu, which will need to be managed,” says
Commissioner Donald Couch. “A tsunami generated in a South
American earthquake will take several hours to get here,
which gives us some warning, but if a tsunami is triggered
in the Kaikoura Canyon, there will be only minutes to get to
higher ground.”

Mr Couch says the reports will be used
as part of civil defence emergency preparedness in Kaikoura.
An opportunity to discuss the reports will be given at the
Kaikoura District Council Community Preparedness/Tsunami
Information road show on March 19 and 20.

“I’m pleased
that the findings of these reports are being shared with the
local community,” Mr Couch says. “Knowing more about the
risk means the community knows what to expect and can
therefore be better prepared.”

The distant-source report
says the South American-generated tsunami would flood parts
of Kaka and Weka Streets in South Bay, and Fyffe and Wakatu
Quays in Kaikoura.

The second wave would be the largest,
travelling at around a metre a second, the report says. The
modelled wave was based on the 1868 Aria (South America)
tsunami, the largest inundation historically, under the
assumption it arrived at high tide.

The second report,
into the stability of the sea floor at the top of the
Kaikoura Canyon, concludes that there is potential for large
landslides to occur in shallow water, very near the coast to
the south of Kaikoura Peninsula.

“This is a unique
situation in New Zealand, where a very shallow (as little as
30 metres deep) canyon edge is less than a kilometre from
the coast in an area with coastal populations and nationally
critical infrastructure,” says Dr Joshu Mountjoy, NIWA
marine geologist and project leader. “This research builds
on earlier work in the area using state-of-the-art tools
that allow us to really see what is happening on the sea
floor, and in the sediment and to assess the
landslide-tsunami hazard”.

Such landslides are expected
to occur during or after an earthquake and could generate a
tsunami that would arrive in Kaikoura very quickly. “If a
large earthquake is felt in the area, that is the most
likely warning sign that a tsunami could eventuate,” Mr
Couch says. “If Kaikoura coastline residents feel an
earthquake so strong they can’t stand up or one that goes
on for more than a minute, then move to higher ground. And
do the same if you see the sea suddenly going out or rushing
in.”

The first phase of the Kaikoura Canyon study has
identified areas of sediment around the canyon rim that
could fail during an earthquake, and demonstrates that this
is a very active area of sediment movement into the Kaikoura
Canyon.

“We have had to re-evaluate previous models for
a landslide-generated tsunami here, but have identified
several locations where shallow-water sediment bodies at the
edge of Kaikoura Canyon pose a potentially hazardous
scenario,” says Dr Joshu Mountjoy.

“It’s very
important to understand how big these tsunamis could be, and
to determine where and when they might occur,” he
says.

In February this year, Dr Mountjoy and the research
team were working on NIWA’s research vessel Ikatere
off the Kaikoura Coast on the second phase of the Kaikoura
canyon project, collecting a large amount of geotechnical
data. This will be used to determine the size of potential
failures and computer simulations will assess the stability
of the area during strong earthquakes. This work will
underpin further tsunami modelling as part of Phase-3 in
2015.

This local-source tsunami report is in response to
the findings of a 2004 report by NIWA for Environment
Canterbury, which describes the actual make-up of the sea
floor and identifies areas that could start a submarine
landslide and tsunami for further
investigation.

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